In recent years, wishbone suspensions and MacPherson strut suspensions have been much used as suspensions for automobiles. Among wishbone suspensions, a double wishbone suspension supports a wheel and a tire through a link mechanism having a parallelogram shape constituted by an upper arm and a lower arm. With a double wishbone suspension, when the wheel moves up and down due to input from the road surface, the kingpin inclination and the camber angle do not readily deviate, making it easy to stably maintain the area of contact between the tire and the road surface. A multi-link suspension which is a variation on a double wishbone suspension is also much used as a suspension for automobiles.
An arm material for an upper arm (also called an upper control arm) and a lower arm, which are structural components of a double wishbone suspension and a multi-link suspension (collectively referred to in this description as a “double wishbone suspension”), has heretofore been manufactured by welding of parts formed by press working of a steel sheet, by punching of a thick plate, by forging of an aluminum alloy, and similar methods.
In recent years, decreasing the weight of automobiles has been promoted as a countermeasure against global warming. A double wishbone suspension tends to become larger in structure than a MacPherson strut suspension, so its weight tends to increase. For this reason, there is a strong demand for further decreases in weight, decreases in size, increases in strength, and decreases in the cost of structural parts such as upper arms and lower arms of double wishbone suspensions.
Patent Document 1 discloses an invention pertaining to a wishbone suspension having an upper arm which is formed into the shape of an A by bending of a single metal sheet and which has an upper portion, an inner flange, and an outer flange. A ball joint support portion is provided at the outer end in the widthwise direction of the vehicle body on the upper portion of the arm. A bush support portion is provided on the inner end in the widthwise direction of the vehicle body on the inner flange. The inner flange and the outer flange are formed so that on the outer side in the widthwise direction of the vehicle body, the length in the vertical direction of the outer flange is longer than the length in the vertical direction of the inner flange. In addition, the inner flange and the outer flange are formed so that on the inner side in the widthwise direction of the vehicle body, the length in the vertical direction of the inner flange is longer than the length in the vertical direction of the outer flange. That invention can reduce the weight and increase the stiffness of an upper arm.
That upper arm is manufactured by bending a base material comprising a single metal sheet into the shape of an A. Therefore, manufacturing costs are necessarily high, and there is unavoidably a bending limit. This makes it difficult to form a shape which is ideal from the standpoint of performance, and yield is poor.
In addition, there are demands for further increases in the strength of this type of upper arms.
Patent Document 2 discloses an arm for a suspension having a double tube structure in locations requiring strength. According to that invention, a suspension arm is manufactured by fitting a reinforcing pipe member at locations requiring strength, and carrying out drawing, hydroforming, and press forming to obtain a final form. That manufacturing method requires complicated working steps, so the yield of a product is poor and it is difficult to reduce costs. In addition, it has the problem that strength is markedly decreased in locations which are not reinforced.
In Patent Document 3, the present applicant disclosed an invention relating to a bending apparatus. FIG. 3 is an explanatory view schematically showing that bending apparatus 0. In that invention, as shown in FIG. 3, a product formed by bending (a bent product) is manufactured using a bending method in which a metal pipe or a metal rod (collectively referred to below as a metal pipe) 1 which is supported by a support means 2 so as to be able to move in its axial direction undergoes bending on the downstream side of the support means 2 while being fed from the upstream side to the downstream side by a feed device 3.
Namely, a high frequency heating coil 5 rapidly locally heats the metal pipe 1 on the downstream side of the support means 2 to a temperature at which quenching is possible. On the downstream side of the high frequency heating coil 5, a water cooling device 6 rapidly cools the heated metal pipe 1. A movable roller die 4 has at least one pair of rolls 4a which can support the metal pipe 1 as it is being fed. The movable roller die 4 imparts a bending moment to the portion of the metal pipe 1 which was heated to a high temperature by moving two-dimensionally or three-dimensionally on the downstream side of the cooling device 6, thereby performing bending of the metal pipe 1. According to that invention, a bent product can be manufactured with a high operating efficiency while achieving a desired working accuracy.